A hydrometer is a common way of measuring the specific gravity of a liquid. The classic hydrometer is an instrument that typically finds its place on a lab bench. It is composed of glass and consists of a cylindrical stem, weighted with mercury or lead shot to make it float upright, and the upper stem is marked with a scale, so that the specific gravity can be read directly. The classic hydrometer is used by collecting a sample of liquid of unknown specific gravity, pouring it into a graduated cylinder or a similar container, the hydrometer is then placed to free float in the cylinder, and height of the water relative to the hydrometer is measured by the calibrated marks on the stem. The problem with the traditional hydrometer is that it cannot provide in situ measurements, it is fragile, it is time consuming to take measurements, and it does not provide electronic data.
Another type of hydrometer is the swing arm hydrometer. There are a series of swing arm hydrometers that are self-contained hand held instruments and others that measure in situ. The swing arm is a flotation device that rotates around a fixed axis in response to the buoyant force of the liquid being measured. The portable swing arm hydrometer simply consists of a box, a swing arm and a counter weight. The specific gravity of a sample liquid may be taken with the portable version on site but a sample must be removed, sampled and disposed of. The in situ type of swing arm hydrometer does not require the removal of a sample of liquid for testing. The problem with the swing arm hydrometer is that it is hard to install, hard to clean, and cumbersome to use.
Another type of hydrometer floats freely in a liquid to provide in situ specific gravity measurements. This hydrometer consists of a weighted float, a horizontal arm and a scale. The buoyant force of the liquid causes the hydrometer to float at varying vertical tilts. The change in tilt is measured by a combination of a mechanical arm that moves in relationship to the weighted float and a scale that reports the specific gravity. The problem with this hydrometer is that it requires a mechanical arm to be attached to the hydrometer.